1. Field of the Invention
The present invention relates to a work vehicle and a control method therefor.
2. Background Information
A power-transmission device (referred to hereinbelow as a “torque converter-type transmission”) having a torque converter and a multi-stage speed change gear is well known as a work vehicle such as a wheel loader. However, recently hydraulic mechanical transmissions (HMT) have become known as power-transmission devices in place of torque converter-type transmissions. As disclosed in Japanese Laid-Open Patent Publication No. 2006-329244, an HMT has a gear mechanism and a motor connected to rotation elements of the gear mechanism, and a portion of the driving power from the engine is converted to hydraulic pressure and transmitted to a travel device, and the remaining portion of the driving power is mechanically transmitted to the travel device.
The HMT is provided with a planetary gear mechanism and a hydraulic motor, for example, in order to allow stepless speed variation. A first element among the three elements of a sun gear, a carrier, and a ring gear of the planetary gear mechanism is coupled to an input shaft, and the second element is coupled to an output shaft. A third element is coupled to the hydraulic motor. The hydraulic motor functions as either a motor or a pump in response to the travel state of the work vehicle. The HMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the hydraulic motor.
Further as described in Japanese Laid-Open Patent Publication No. 2008-247269, an electric-mechanical transmission device (EMT) has been proposed as a technique similar to the HMT. An electric motor is used in the EMT in place of the hydraulic motor in the HMT. The electric motor functions as either a motor or a generator in response to the travel state of the work vehicle. Similar to the HMT, the EMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the electric motor.